1. Field of the Invention
The present invention relates to a cushion plate used as a buffer in a multi-plate frictional engagement element provided in, for example, an automatic transmission of a vehicle. Specifically, the present invention relates to a cushion plate in which stress concentration is reduced when it is deflected.
2. Description of the Related Art
Vehicular automatic transmissions utilize a multi-plate clutch or a multi-plate brake for changing a power transmission path within the transmission mechanism. In the multi-plate clutch or brake, movement of a piston of a hydraulic servo (actuator) is controlled based on the pressure of oil supplied from, for example, a hydraulic (oil pressure) controller, thereby pressing or releasing a friction plate unit and engageing or disengaging the clutch or brake. The clutch or brake has a cushion plate shaped like a belleville-spring between the piston and the friction plate unit. When the clutch or the brake is engaged, the cushion plate is deflected to absorb engagement shock (see, for example, Japanese Patent Application Laid Open No. 10-246249).
However, the cushion plate may be dragged with the friction plate of the clutch, the brake or the like, by flow of the lubrication oil in the direction of rotation. Furthermore, for example when the cushion plate is used in a clutch, it may rotate relative to the clutch drum. That is, the cushion plate may be abraded undesirably by friction with an adjacent member rotating relative to the cushion plate.
Therefore, it is preferable that, for example, pawls 101b be integral with and extend from the body 101a of the cushion plate 101 as shown in FIG. 4(a) and FIG. 4(b), and that the friction plate 41a adjacent the cushion plate 101 be in spline engagement with the cushion plate 101 via a spline 4s, as shown in FIG. 5(a).
However, as shown in FIG. 5(b), when piston 43 of a hydraulic servo moves in the direction indicated by arrow A, the belleville-spring shaped cushion plate 101 is pushed toward the adjacent friction plate 41a, and is deflected. As the cushion plate 101 is compressed in this manner, it is extended radially outward as indicated by arrow B, and the plate body 101a receives a turning force as indicated by arrow ω1-ω2 shown on FIG. 4(b). In other words, a circumferential direction tensile stress is generated in the outer peripheral portion of the cushion plate 101. Thus, a maximum stress σymax, greater than the average stress σyn, is generated as a concentrated stress shown as stress distribution of a cross section y-y in FIG. 4(b). In other words, a relatively large stress concentration occurs at base portions 101g of the pawls 101b, adversely affecting durability.
Furthermore, as shown in FIG. 6, for example, if an arc at a base portion 101g′ of a cushion plate 101′ is enlarged in order to reduce stress concentration, the base portion 101g′ may undesirably contact a corner 4sa of the spline 4s, thereby reducing the contact area between side surface 101f′ of a pawl 101b′ and a side surface 4sb of the spline 4s. Such a small contact area is undesirable when, for example, the cushion plate 101′ receives a large rotational force.
Moreover, as shown in FIG. 7(a) and FIG. 7(b) for example, if a plate body 201a of a cushion plate 201 has recesses 201c extending radially inward at a base portion 201g of pawls 201b, it is possible to increase the size of an arc at the base portion 201g. However, since tensile stress is generated in the circumferential direction as indicated by the arrow ω1-ω2, a maximum stress σzmax (which is larger than the maximum stress σymax) and average stress σzn, are created in an area of stress concentration shown as a cross section z-z. In other words, a relatively large concentration of stress occurs at the recess portions 201c. 